devel/rubygem-docopt: Add new port

Rubygem implementation of docopt.

Docopt helps you:

• define an interface for your command-line app, and
• automatically generate a parser for it.

Docopt is based on conventions that have been used in help messages for
decades and man pages for describing a program's interface. An
interface description in docopt is such a help message, but formalized.

WWW: https://github.com/docopt/docopt.rb

Reviewed by: ruby (sunpoet)
Approved by: gerald (mentor)
Differential Revision: https://reviews.freebsd.org/D35031